Magnetic tape data storage typically provides one or more prerecorded servo tracks to allow precise positioning of a tape head with respect to those prerecorded servo tracks. Servo elements or sensors disposed on the tape head are used to track the recorded servo tracks. The tape head comprises one or more read/write elements precisely positioned with respect to those servo sensors. One example of a magnetic tape system is the IBM 3590, which employs magnetic tape having prerecorded servo patterns that include three parallel sets of servo edges, each servo edge being an interface between two dissimilar recorded servo signals, each set of servo edges comprising one servo edge on each of opposite lateral sides of a middle recorded servo signal.
In certain embodiments, the tape head includes a plurality of servo sensors for each servo edge, with the result that the tape head may be stepped between those servo sensors, each positioning the read/write elements laterally at different interleaved groups of data tracks. Typically, for a given servo pattern of a set of two servo edges, the outer servo signals are recorded first and the center servo signal is recorded last to provide the servo edges. The nominal separation distance between the servo edges of each set of servo edges is a certain distance, but there is variation in the magnetic separation between the servo edges, for example, due to the variation of the width of the physical write element which prerecords the servo pattern, due to variation in the magnetic characteristics of the physical write element, etc. The variation may occur between servo tracks in a single magnetic tape, and may occur between prerecording devices and therefore between magnetic tapes.
To reduce the apparent difference of the edge separation distance of the prerecorded servo tracks from nominal, the prerecording of the servo tracks is conducted at different amplitudes so as to attempt to compensate for the physical difference and provide a magnetic pattern that is closer to nominal. Thus, the difference in physical distance and the amplitude compensation may tend to offset each other with respect to the apparent distance between the servo tracks. These actions may provide an adequate signal for track following at the servo edges.
However, to increase track density, a servo sensor may be indexed to positions laterally offset from the linear servo edges to provide further interleaved groups of data tracks. The indexed positions are determined by measuring the ratio between the amplitudes of the two dissimilar recorded servo signals. Thus, when the amplitudes of the recorded servo signals are varied to compensate for physical distance variations, track following the prerecorded servo edges at the offset indexed positions becomes less precise. As the result, the data tracks may vary from the desired positions, i.e. be “squeezed” together, such that writing on one track with a write element that is subject to track misregistration (TMR) may cause a data error on the immediately adjacent data track.
The tape path of the above IBM 3590 is a guided tape path. In such a guided tape path embodiment, the magnetic tape can be moved in a first direction and an opposing second direction along a first axis, i.e. along the longitudinal axis of the tape. Movement of that tape along a second axis orthogonal to the first axis, i.e. the lateral axis of the tape, is minimized. Limiting the lateral movement of the magnetic tape results in reducing noise.
Another approach, however, is required for open channel guiding in which the magnetic tape can move laterally a distance which is substantially greater than the separation between index positions, thereby introducing substantial noise into the guiding process. The guiding signal to noise ratio thus becomes negative, with the guiding noise being far larger than the step from one ratio to another, making it difficult to gather data points with a monotonic slope to conduct a calibration of the servo ratios.
In some embodiments, the tape head comprises two head modules, typically referred to as left and right head modules, adjacent to each other and each including write, read and servo elements. To accommodate read-after-write data recording, a write element on one head module will be paired opposite a read element on the other head module. With this configuration, data which is written to the tape can be immediately read and checked for errors. If an error occurs, the data can be quickly rewritten.
In some embodiments, tracking is maintained using the servo elements on the same head module as the active read elements. Thus, if data is being recorded by the write elements on the left head module and read by the read elements on the right head module, the servo elements on the right head module is used for tracking. Such a configuration introduces certain inaccuracies due to “special filtering” caused by the distance, however small, between the two head modules. More recent embodiments have reduced the inaccuracies by using the servo elements on the same head module as the write elements, called “same-gap servoing.”
Same-gap servoing, however, introduced other noise in the signals generated by the servo elements. Such noise is caused by the close proximity of the write signals transmitted to the write elements to the servo signals generated by the servo elements as they read the servo patterns from the tape. One method that has been used to overcome noise from same-gap servoing is to use a different cable and module layout to increase the separation between the write signals and the servo signals. However, redesigning the cable and module configurations is complex, expensive and does not allow the use of existing hardware components.